My situation is a bit different as I'm running Unraid under ESXi, which isn't supported, but has done exactly what I need it to do for years.

In my case, everything works perfectly using ESXi 6.0 and ESXi 6.5 but as soon as I tried upgrading to ESXi 6.7 I kept getting the "VFS: cannot open root devise "(null)" or unknown-block(0,0): error -6" message.

I tried everything I could think of and had basically decided that ESXi 6.5 was the highest I could use while keeping Unraid. I added the " root=sda" on each of the append initrd lines, rebooted the VM, and it booted up perfectly.

Honestly, I have no idea what this does or why it would make any difference at all, but it fixed booting an Unraid 6.6.6 VM in ESXi 6.7